1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a multi-stage automatic transmission that is mounted, for example, in a vehicle, and in particular, relates to a hydraulic control apparatus for a multi-stage automatic transmission that ensures the ability of the vehicle to travel during an all-solenoids-off failure.
2. Description of the Related Art
Conventionally, a staged automatic transmission that is mounted, for example, on a vehicle enables multi-speed shifting by controlling the engagement state of a plurality of friction engagement elements (clutches and brakes) by using a hydraulic control apparatus and forming transmission paths in the speed change mechanism at each shift speed. This hydraulic control apparatus is provided with a plurality of switching valves and regulating valves, and is also provided with a plurality of solenoid valves for electrically controlling the operation of these valves. The control of the multi-speed shifting as described above may be carried out by actuating these solenoid valves.
However, in such a hydraulic control apparatus described above, a state in which electrical signals from the solenoid valves are not sent, which is referred to as a all-solenoids-off fail state, may occur when, for example, there is a severed wire or short, or when, for example, some type of failure is detected in the hydraulic control apparatus. Enabling the establishment of shift speeds by hydraulic control in order to guarantee the ability of the vehicle to travel such a state has been proposed (refer to Japanese Patent Application Publication No. 2004-28277).
This device is structured such that even if an all-solenoids-off fail occurs, for example, during travel in the drive (D) range, when traveling forward in the third forward speed or in the fourth forward speed, travel is locked, for example, at the fourth forward speed, and for example, when traveling forward in the first forward speed or in the second forward speed, travel is locked at the first forward speed. Furthermore, this device is structured so as to change or lock in the first forward speed because of an engine stop after locking in the fourth forward speed.
However, in recent years, with the goal of enhancing the fuel economy of vehicles, the development of multiple speeds (for example, eight forward speeds) in a staged automatic transmission has been progressing, and in this multi-stage automatic transmission, the shift speeds are structured so as to be segmented into broad gear ratios ranging from a low gear ratio to a high gear ratio. In such a multi-stage automatic transmission, during the all-solenoids-off fail while traveling as described above, there is a concern that downshifting of two or more stages (for example, from fifth to third speed) may occur due to separating and locking the shift speed at two predetermined stages (relatively high and low speeds), and downshifting of two or more steps occurring when this is not intended or preferred by the driver. However, simply locking at the high speed makes restarting the vehicle after temporarily stopping the vehicle difficult, and there is a concern that a vehicle that is experiencing failure will not be capable of self-propulsion if it is simply locked at a high speed.